2 EXPERIMENTAL
2.1 Structural design on the nanometer level
2.1.2 Employed precursors
2.1.2.1 Q8M8-cube precursor solution
Synthesis: (modified from [125]) An aqueous alkaline silicate solution was prepared by mixing 0.0448 mol 25% tetramethylammonium hydroxide (TMAOH in CH3OH) + 0.224 mol CH3OH + 0.392 mol H2O + 0.0448 mol tetramethyl orthosilicate (TMOS). The solution was stirred overnight under argon atmosphere and afterwards kept at 4 °C. Due to the highly alkaline conditions the silica does not polymerize but reacts to double four rings (see Figure 2.1.5).
Figure 2.1.5. Synthesis of the Q8M8 Si-precursor and the further preparation of a silica/surfactant liquid crystal. The solution is stabilized through the highly alkaline conditions.
Processing of a highly alkaline silica/surfactant liquid crystal:
A silicate/surfactant liquid crystal was prepared by adding a 7-12%(w/w) aqueous solution of the cationic surfactant cetyltrimethylammonium bromide (CTAB) to the aqueous alkaline silicate solution (Q8M8) at RT. The mixture separates into a viscous silicate/surfactant-rich LC phase with hexagonal or lamellar structure and a water-rich phase. The highly alkaline conditions (pH=12.5) prevent the polymerization of the double-four-ring silicate oligomers.
2.1 STRUCTURAL DESIGN ON THE NANOMETER LEVEL
Si O O
O O
CH3
CH3 CH3
C H3
Si
O O
O O OH
O H
O H OH OH
O
+ 4 H
EGMS
Ar, 140 °C
2.1.2.2 Ethylene glycol-modified silane (EGMS)
A main problem in the fabrication of monolithic silica materials derived from conventional alkoxysilanes is the release of alcohols due to hydrolysis and condensation reactions during the process. In the presence of methanol or ethanol the solvent phase becomes more lipophilic. Therefore, the hydrophobic interactions of the surfactant molecules are destabilized. As a consequence, the formation of a lyotropic mesophases is in many cases prohibited. To overcome this, the conventional silica precursor tetraethylorthosilicate (TEOS) was modified with ethylene glycol.
Synthesis of ethylene glycol-modified silane:
In a synthesis described by Mehrotra [126] the ethoxy groups of TEOS were substituted with ethylene glycol at 413 K under argon atmosphere (Figure 2.1.6). Ethanol, which is produced during the transesterification reaction, was continuously removed by distillation over a Vigreux column. At the end of the reaction excess ethanol was removed in vacuo.
The final product, a transparent viscous liquid was analysed by 29Si(H)-HMBC-NMR using CDCl3 as solvent and revealed one peak at about -83 ppm. TGA measurements were performed to determine the Si-content.
Figure 2.1.6. Synthesis of the ethylene glycol modified precursor.
Table 2.1. Theoretical SiO2- and Si-content of ethylene glycol-modified silane (EGMS).
MW / g mol-1 (%(w/w))
SiO2-content /
%(w/w)
Si-content /
%(w/w)
theroretical 272.31 (100) 22.1 10.3
experimental - 20.8 9.7
2.1 STRUCTURAL DESIGN ON THE NANOMETER LEVEL
2.1.2.3 Phenylene-bridged ethylene glycol-modified silane (bPhGMS)
1,4-Bis(triethoxysilyl)benzene was synthesized via Grignard reaction from dibromobenzene as described by Shea et al [68]. TEOS (450 mL, 2 mol), dry tetrahydrofuran (THF, 250 mL), and a few crystals of iodine were added in a flame-dried Ar-purged three-necked round-bottomed flask with a magnetic stirrer and magnesium turnings (15 g, 0.62 mol). The solution was brought to reflux (65 °C) and 1,4-dibromobenzene (48 g, 0.2 mol) in dry THF (125 mL) was added dropwise during a 2 h period. The reaction was allowed to reflux for another 1.5 h, becoming greyish yellow.
While cooling to room temperature the THF was removed by vacuum evaporation. The precipitated MgBr was removed by vacuum filtration. Subsequently, about 200 mL petroleum ether were added to precipitate the residual MgBr, which was again removed by filtration.
Figure 2.1.7 Synthesis of the phenylene-bridged ethylene glycol modified precursor.
The final product was obtained after removing excess TEOS (65 °C, 0.5 Torr) and appeared as colourless oil. The product was analyzed by 1H NMR (1.26 ppm(t, J=6.97 Hz, CH3), 3.89 ppm (q, J=6.97 Hz, OCH2), 7.69 ppm (s, CH); Si: -59.0 ppm). Recovery yield was between 19 and 42% compared to the theoretical value (80.53 g).
2.1 STRUCTURAL DESIGN ON THE NANOMETER LEVEL
In a second step 1,4-b(triethoxysilyl)benzene was reacted with ethylene glycol in a molar ratio of 1:6, otherwise the same procedure was followed as described for EGMS. The final product, this time consisting of transparent solid grains, was characterized with TGA to determine the Si-content (see Table 2.2). No NMR-measurements could be performed due to the low solubility of the precursor in other solvents than alcohol or water. 13C and
29Si CP MAS NMR studies were performed on the final dried gels.
Table 2.2: Theoretical SiO2- and Si-content of phenylene-bridged ethylene glycol-modified silane (bPhGMS).
MW / g mol-1 (%(w/w))
SiO2-content /
%(w/w)
Si-content /
%(w/w)
theoretical 498.64 (100) 24.1 11.3
experimental bPhGMS01 24 11
bPhGMS02 22 10
bPhGMS03 15 7
2.1.2.4 Preparation of monolithic gels
Gels were made with different surfactants as structure directing agents and at various pH values. Other parameters that were changed in the experiments are the weight-ratio of Si/surfactant, Si/solvent and the temperature. As will be demonstrated, these parameters (type of precursor and surfactant, pH-value, temperature, dilution) have a strong influence on the formation of the mesostructure and the macromorphology, and therefore allow to some extent a control over the pore structure.
In a typical procedure, a given amount of surfactant was homogeneously dissolved in an aqueous solution of hydrochloric acid (pH=0-7). For the sol-gel synthesis the resulting LC-phase was added to the precursor under stirring at RT. After homogenization using a vortex stirrer, ultrasound or a centrifuge, the resultant liquid mixture was poured into containers. The container was sealed and kept at 313 K (unless denoted otherwise) for gelation. After gelation, the wet gels were kept at a temperature of 313 K for another 7 days for aging.
The denotation of samples prepared with the glycol-modified precursors in the presence of Pluronic P123 is explained in Figure 2.1.8.
2.1 STRUCTURAL DESIGN ON THE NANOMETER LEVEL
e.g.: E 80 2
Precursor ->E… EGMS bPh … bPhGMS
Figure 2.1.8. Denotation of the samples prepared with ethylene glycol-modified silane (EGMS) and bridged-phenylene glycol-modified silane (bPhGMS) and Pluronic P123 as structure directing agent. The weight ratio of P123/HCl (aq.) was kept constant at 30/70 unless denoted.
2.1.3 Drying of the wet monoliths
2.1.3.1 Supercritical drying
Samples prepared with EGMS/P123, bPhGMS/P123 and EGMS/Brij 97, were dried with carbon dioxide or methanol as supercritical fluids. The critical parameters of the two solvents are shown in Table 2.3. As obvious, the gels dried with methanol are exposed to higher temperatures throughout the process. In both cases the pore fluid first was exchanged to methanol (washing for 4 times).
CO2 Supercritical drying was performed at the planta pilota at the MATGRAS (Barcelona, Spain), as well as in a laboratory autoclave. The aged and washed monoliths were carefully put into the autoclave and covered with methanol. The sealed container was flushed with liquid CO2 (p=60-100 bar). After complete solvent exchange (for details see appendix), the system was sealed again and the temperature was increased above Tc, simultaneously leading to a pressure rise above pc. The supercritical CO2 was then removed carefully.
Supercritical drying with methanol has the advantage of being a fast process because no time-consuming solvent exchange to CO2 has to be performed. Of course, the high pressure and temperature applied have more influence on the network so that a restructuring of the network may occur. Also functional organic groups may react, be replaced or destroyed. The supercritical drying was performed in a high-temperature and high-pressure plant. Therefore, the samples were covered with methanol and subsequently the pressure was increased to 52 bar. After that the temperature was increased over Tc
within ~2 h (see appendix), while the pressure increased to 140 bar. After 4 h in the supercritical state, methanol was removed within 70 min and the autoclave with the samples was subjected to natural cooling. Crack-free monoliths were obtained.
2.1 STRUCTURAL DESIGN ON THE NANOMETER LEVEL
Table 2.3 The two different supercritical fluids employed in the supercritical drying of the monolithic gels.
Solvent Notation Tc / °C pc / bar Advantage Disadvantage CO2 -scd(CO2) 31.3 73 low Tc
not flammable
time-consuming solvent exchange
CH3OH -scd(MeOH) 240 81 fast high temperatures and pressures flammable solvent
Whereas gels dried with CO2 are known to be rather hydrophilic, gels dried with methanol are more hydrophobic (surface covered with methoxy groups) [127].
2.1.3.2 Ambient pressure drying after surface silylation with trimethylchlorosilane
The liquid crystalline template was removed by surface silylation with trimethylchlorosilane. Therefore the wet gel was reacted by immersing the gel body in a solution of 10 (v/v) trimethylchlorosilane in petroleum ether (PE) for 12 h and afterwards washed 3 times with PE and 5 times with ethanol to remove unreacted silane species (samples are denoted as -tms). The surface modified samples were dried by slowly heating to 55 °C, 70 °C and 120 °C (up to 60 °C the heating rate was 3 °C/2 h, after that 10 °C/2 h).